linux-kernel - Re: [PATCH] counter: drop chrdev

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Date:   Mon, 18 Oct 2021 11:14:15 -0500
From:   David Lechner <david@...hnology.com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] counter: drop chrdev_lock

On 10/18/21 4:51 AM, William Breathitt Gray wrote:
> On Mon, Oct 18, 2021 at 11:13:16AM +0200, Greg KH wrote:
>> On Mon, Oct 18, 2021 at 05:58:37PM +0900, William Breathitt Gray wrote:
>>> On Mon, Oct 18, 2021 at 08:08:21AM +0200, Greg KH wrote:
>>>> On Sun, Oct 17, 2021 at 01:55:21PM -0500, David Lechner wrote:
>>>>> This removes the chrdev_lock from the counter subsystem. This was
>>>>> intended to prevent opening the chrdev more than once. However, this
>>>>> doesn't work in practice since userspace can duplicate file descriptors
>>>>> and pass file descriptors to other processes. Since this protection
>>>>> can't be relied on, it is best to just remove it.
>>>>
>>>> Much better, thanks!
>>>>
>>>> One remaining question:
>>>>
>>>>> --- a/include/linux/counter.h
>>>>> +++ b/include/linux/counter.h
>>>>> @@ -297,7 +297,6 @@ struct counter_ops {
>>>>>    * @events:		queue of detected Counter events
>>>>>    * @events_wait:	wait queue to allow blocking reads of Counter events
>>>>>    * @events_lock:	lock to protect Counter events queue read operations
>>>>> - * @chrdev_lock:	lock to limit chrdev to a single open at a time
>>>>>    * @ops_exist_lock:	lock to prevent use during removal
>>>>
>>>> Why do you still need 2 locks for the same structure?
>>>>
>>>> thanks,
>>>>
>>>> greg k-h
>>>
>>> Originally there was only the events_lock mutex. Initially I tried using
>>> it to also limit the chrdev to a single open, but then came across a
>>> "lock held when returning to user space" warning:
>>> https://lore.kernel.org/linux-arm-kernel/YOq19zTsOzKA8v7c@shinobu/T/#m6072133d418d598a5f368bb942c945e46cfab9a5
>>>
>>> Instead of losing the benefits of a mutex lock for protecting the
>>> events, I ultimately implemented the chrdev_lock separately as an
>>> atomic_t. If the chrdev_lock is removed, then we'll use events_lock
>>> solely from now on for this structure.
>>
>> chrdev_lock should be removed, it doesn't really do what you think it
>> does, as per the thread yesterday :)
>>
>> So does this mean you can also drop the ops_exist_lock?
>>
>> thanks,
>>
>> greg k-h
> 
> When counter_unregister is called, the ops member is set to NULL to
> indicate that the driver will be removed and that no new device
> operations should occur (because the ops callbacks will no longer be
> valid). The ops_exist_lock is used to allow existing ops callback
> dereferences to complete before the driver is removed so that we do not
> suffer a page fault.
> 
> I don't believe we can remove this protection (or can we?) but perhaps
> we can merge the three mutex locks (n_events_list_lock, events_lock, and
> ops_exist_lock) into a single "counter_lock" that handles all mutex
> locking for this structure.
> 

The different mutexes protect individual parts of the counter struct
rather than the struct as a whole (a linked list, kfifo reads, and
callback ops), so I think it makes the code clearer having individual
mutexes for each rather than having a global mutex for unrelated
actions.